1. Field of Disclosure
The present disclosure of invention relates to the field of semiconductor device fabrication, and more specifically, to a method of forming magnetic tunnel junction (MTJ) structures.
2. Description of the Related Technology
With function densities (e.g., memory densities) in semiconductor devices tending to become higher and higher, it has been proposed that magneto resistive random access memories (MRAMs) which utilize magnetic tunnel junction (MTJ) structures as their elemental storage unit be used for next generation semiconductor memories. Such MRAMs promise to provide many advantages, for example in terms of greater write/read speeds, lower power consumption and longer device lifetime. In the elemental MTJ storage units (storage elements), a magneto-resistance ratio varies according to the respective magnetization directions of two respective ferromagnetic layers. A determination as to whether a logic “1” or logic “0” is being stored in the MTJ element can be made by measuring a change in voltage or in current of a sensing signal where this change is due to the magneto-resistance ratio variation.
The MTJ elements can have very small sized structures. More specifically, and as described for example in U.S. Patent Publication No. 2011/0129946 A1 (Zhong et al., “High density spin-transfer torque MRAM process”) where said early publication is incorporated herein by reference, the MTJ element typically has a critical dimension that is less than 100 nm and typically has a thickness of less than 5 nm. Therefore, when mass production of devices containing such small MTJ element is considered, it becomes a problem as to how to accurately control MTJ dimensions within the context of semiconductor device fabrication, especially if it is desired to further decrease MTJ sizes in the near future.
Referring to FIG. 1 sake of background, illustrated there is a semiconductor device structure containing MTJ elements which are formed in accordance with a method disclosed in the above-cited U.S. Patent Publication No. 2011/0129946 A1. In the method, and as a foundational start, so-called, metal landing pads 11 made of Cu are formed in a substrate and then intermediate via contacts (VAC) 21 also made of Cu are formed. Next, a metal separation (VAM) dielectric layer 14a and via-covering VAM pads 31p are formed thereon. Then, a MTJ stack is blanket formed on the metal separation (VAM) dielectric layer 14a and on the VAM pads 31p. After that, selective etching is performed using a hard mask disposed on the MTJ stack's top layer to thereby form the illustrated MTJ elements 51. Finally, a chemical mechanical planarization (CMP) process is performed on the resulting structure containing the formed MTJ elements 51 for further forming connection lines 91 made of Cu.
However, in the process disclosed by US 2011/0129946, the hard mask consists of Ta, and the use of such a Ta based hard mask can result in etching loss. More specifically, during the etch process, a small portion of the sidewalls and the upper corners of the hard mask layer tend to become etched out, thus altering the pattern defined by the hard mask as the etch process proceeds. Therefore, it becomes difficult if not impossible to provide an exact pattern transfer to the MTJ element 51. Additionally, if a planarizing CMP is performed after the MTJ elements 51 are formed by the Ta based etch process, the post-etch CMP may cause damage to the very thin MTJ elements and degrade performance or render the device inoperable.
In view of this, there is a need for a different method of forming MTJ elements where the alternative method is capable of more precisely defining MTJ element sizes while preventing MTJ elements from being damaged by a post etch planarization process.
It is to be understood that this background of the technology section is intended to provide useful background for understanding the here disclosed technology and as such, the technology background section may include ideas, concepts or recognitions that were not part of what was known or appreciated by those skilled in the pertinent art prior to corresponding invention dates of subject matter disclosed herein.